1. Field of the Invention
This invention relates to single lens reflex cameras, and more particularly to a camera which is equipped with a main movable mirror for cooperation with a view-finder which has at least a portion of the area thereof capable of passing light therethrough and an auxiliary movable mirror coupled to the main movable mirror in collapsible relation thereto and arranged upon being set in an upright position to reflect the light passed through the main movable mirror to a light sensor, or the like.
2. Description of the Prior Art
In the art of single lens reflex cameras, for example, there have been proposed in the past a wide variety of methods and apparatus for splitting the light passing through a photographic objective lens into parts one of which is conducted to a photosensitive element for focus detection whereby the focus condition of the photographic objective is displayed in the field of the view-finder, or whereby automatic focusing of the photographic objective may be achieved.
Installation of such a photosensitive element for focus detection within the camera body imposes limitations on the optics and/or limitations on the structure of the mirror mechanism since the photosensitive element is often located on the bottom of the mirror box. Therefore, in order to conduct the light passed through the objective lens to the photosensitive element, the view finder main mirror is usually made partially reflective and an auxiliary mirror is positioned behind the main mirror to direct the light passed through the main mirror to the photosensitive element.
However, the use of the auxiliary mirror constituting part of the focus detecting optical system at such a location lying in the optical path to the photographic film necessarily leads to the requirement that in order to initiate an exposure, prior to opening the shutter, both the view-finder main mirror and the auxiliary mirror must be retracted from the photographic light path. Thus, soon after the termination of the exposure, or soon after the shutter is closed, these mirrors must be returned to their initial position. For this purpose, a suitable drive mechanism must also be incorporated into the interior of the camera.
A conventional method for driving this type of arrangement of the main movable mirror and the auxiliary movable mirror, may take, for example, a form such that the main and auxiliary mirrors are simultaneously driven to move. In this form, however, there is a need for a large driving torque in operating both of the movable mirrors. Particularly when the size of the auxiliary mirror is increased, a very strong spring is necessary along with a sufficient reinforcement of each constituent part. This involves a substantial increase in the bulk and size of the camera mechanisms.
The existence of a need for increasing the size of the aforesaid auxiliary mirror will be seen from the fact that in order to improve the accuracy of the focus detecting device, the amount of light taken into the optical system for focus detection tends to increase.